We really have to push the bounds of our capabilities to make rare isotopes in order even to be able to ask the questions that we need to ask.
The question seems straightforward enough: How is matter created? Within that one question are endless possibilities for study, making physics a discipline that Matthew Amthor calls "a team sport."
The game begins with a question. For example: What is happening inside a neutron star as it experiences an x-ray burst, the most common thermonuclear explosion in the galaxy? "X-ray bursts happen all the time because they are not quite as dramatic as supernovae," says Amthor. "Rather than being completely destructive, like a supernova, x-ray bursts just explode on the surface of a neutron star." Because of their repetitive nature, roughly every few hours lasting anywhere from 10 to 100 seconds (as opposed to supernovae, which are observed only about once per century per galaxy), x-ray bursts are accessible for closer, regular study.
"The neutron star itself is such a special beast," says Amthor. "It fits all the mass of a star in such a tiny volume, and basically has the density of an atomic nucleus. So it's like a gigantic nucleus the size of Manhattan, allowing you to see how things interact in ways you can't usually see in a macroscopic system." Amthor, often with the help of students, uses computer models to determine the specific nuclear fusion reactions taking place that drive the bursts. Reproducing the light curve from data collected from x-ray telescopes, he has matched the data so closely that the models agree fairly well. But it's the nuclear physics uncertainties that drive the game to overtime.
Once Amthor has identified an important uncertainty, for example, what particular nuclear reaction rate (many of which are just theoretical educated guesses) can significantly change the x-ray burst, he has to design an experiment and write a proposal to better nail down the reaction rate by producing and studying the rare isotopes involved in the laboratory, such as the National Superconducting Cyclotron Laboratory at Michigan State University — a world leading lab where scientists study rare isotopes.
"A committee of physicists decides among the many proposed experiments what's interesting and may tell us we get one week to study our isotope," he says. "So we make the nucleus of interest and figure out how to make some reaction take place using that nucleus in order to learn the reaction rate that takes place under the conditions in our exploding star."
There's always another question looming on the horizon, which is what drives and excites Amthor, who dreamed from a young age of studying nuclear astrophysics. "We really have to push the bounds of our capabilities to make rare isotopes in order even to be able to ask the questions that we need to ask," he says. "For example, most of the nuclei involved in the r-process — in which most of the elements beyond iron were formed — are still beyond our experimental reach. So when you look at all the isotopes that are imagined to be possible and compare that to everything we can actually make and study in the lab, it shows that we have a limit. We're not able to study all the types of matter that matter in the universe." Not yet, anyhow.
Amthor is pushing on this frontier as well, working with a team of physicists to develop the next generation laboratory for the study of rare isotopes. It's called FRIB, the Facility for Rare Isotope Beams, and preparation for construction is already well under way. "FRIB will be a giant leap forward for the world in nuclear structure and nuclear astrophysics. It will open a whole new landscape for study and shine light on many previously hidden nuclear processes," he says. "I can't wait to see where the results will lead us next."
Posted October 2012